Lubricants, including lubricating oils and greases, are used in a variety of applications in which the lubricant is in contact with or admixed with air often at elevated temperatures and/or under severe operating conditions. Some of these conditions promote thermal and oxidative degradation of the product. This degradation or breakdown is usually manifested by deterioration in appearance, physical properties or in the performance of the lubricant. For example, the formation and deposition of varnishes and sludge on engine surfaces is primarily due to oxidation and polymerization occurring in the lubricating oil. These deposits are undesirable since they contribute to wear and corrosion of the engine surfaces.
Particularly effective lubricant additives because they exhibit desirable anti-oxidant and detergent properties are the zinc salts of esters of dithiophosphoric acid. These compounds are prepared by reacting alkyl or alkaryl compounds with phosphorus pentasulfide and thereafter neutralizing the resulting dithiophosphoric acid with a zinc salt such as zinc oxide to produce the zinc dialkyldithiophosphate additive. While the basic method and variations thereto are well known, the additives produced are not entirely satisfactory. Under conditions of extreme stress, the zinc dialkyldithiophosphate will undergo degradation resulting in the production of sulfides which will pass through the combustion and exhaust system of the vehicle with undesirable effects.
The object of this invention is to provide a novel zinc dialkyldithiophosphate which is more stable under engine operating conditions.